The present invention relates to a system by which the channels for a cross connecting device can be extended without momentary interruption.
A cross connecting device assembles the lines of a plurality of subscribers and connects them to a predetermined trunk line by switching their channels. Generally, an 8.times.8 cross connecting device is provided. In other words, the 8.times.8 cross connecting device switches eight connect them to eight output input channels so as to connect them to eight output channels.
In order to increase the volume of communications handled and to improve the reliability of a communication function, a cross connecting device is required that can the volume of communications number of input channels to a corresponding switch a greater number of input channels to a corresponding number of output channels.
FIG. 11 is a block diagram illustrating as a base set off an 8.times.8 cross connecting device, the arrangement of an 8.times.8 cross connecting device, wherein data are input along eight channels and data are channels. The data that are input and output to eight channels. The data that are input a output are SDH (Synchronous Digital Hierarchy) signals.
SDH signals that are input along the eight channels are transmitted to corresponding interface circuits 10 . . . 17. According to a cell format shown in FIG. 12, the received SDH signals are formed into cells, each of which has a header portion and an information field.
Multiplexers 20 . . . 27 perform high-speed which are output by the conversion of the cell signals, which are output by the respective interface circuits 10 . . . 17, and a of the cells are stacked to form predetermined number of the cells are stacked to form multiplexed cells. Path information is added to the multiplexed cells by VP (virtual path) converters 30 . . . 37, and the resultant data are transmitted to a switching unit 4 of the cross connecting device.
The switching unit 4 determines corresponding channels for the multiplexed cells in accordance with the path information, and switches and outputs them. The multiplexed cells from the switching unit 4 are demultiplexed by demultiplexers 40 . . . 47, respectively, and the cells are converted into the original SDH signals, which are then transmitted to a trunk line via interface terminals 50 . . . 57.
When the switching unit 4 in the 8.times.8 cross connecting device is to be further extended to increase the number of output channels that correspond to the number of input channels, the 8.times.8 cross connecting is normally in the operational state.
In FIG. 13 is shown an exemplary arrangement that is 2 assumed when an expanded set 2 for a switching unit is added to the base set 1 of the 8.times.8 cross connecting device. A switching unit 400 of the expanded set 2 has a 64.times.64 cross connecting function, and includes eight sets of cross connecting structures, each of which is the same as the switching unit 4 of the base set 1.
With the arrangement in FIG. 13, data are input along 64 input channels, the eight input channels of the base set 1 and 56 input channels of the expanded set 2, are cross-connected by the switching unit 400 of the expanded set 2, and are output to 64 output channels from the expanded set 2.
As is shown in FIG. 13, a communication interruption at the input and output sides of the switching unit 4 of the base set 1 is inevitable when the expanded set 2 is added during operation of the base set 1 so as to replace the switching,unit 4 of the base set 1 with the switching unit 400 of the expanded set 2. This interruption occurs during a period extending from the time the switching unit 4 of the base set 1 is disconnected until the expansion using the switching unit 400 of the expanded set 2 is effected. Therefore, transfer paths are severed during this period of time, and data that are to be transmitted are abandoned.
FIG. 14 is a diagram showing another example of an arrangement in which the expanded set 2 for the switching unit is added to the base set 1. Compared with the example shown in FIG. 13, the function of the switching unit 4 of 3 the base set 1 is maintained active, and selectors 60 . . . 67 are provided in the base set 1.
According to this arrangement, the same data are input to the switching unit 4 of the base set 1 and the switching unit 400 of the expanded set 2, and the selectors 60 . . . 67 switch from the base set side to the expanded set side.
With this structure (FIG. 14) the severing of a transfer path, as described relative to FIG. 13, can be prevented. However, because of a difference in processing delay, a phase difference may occur between the base set 1 data and the expanded set 2 data, both of which are transmitted to the selectors 60 . . . 67. Therefore, the data will be abandoned or the data output will be overlapped.